1. Field of the Invention
The present invention relates to plating apparatus in general and in particular to electrolytic plating apparatus for plating selected areas on a continuous metal strip.
2. Description of the Prior Art
Typical of continuous metal strips which are plated using the present invention is a lead frame used in the fabrication of integrated circuit packages.
Selected areas of an integrated circuit package lead frame are plated to obtain certain electrical and mechanical properties. For example, the exposed leads of semiconductor packages are plated with a tin-lead solder to protect the exposed leads from corrosion and to improve the electrical properties of subsequent connections to the leads. Heretofore it was common practice to individually dip the leads of semiconductor packages into a pool of molten solder to coat the leads with solder. However, this solder dipping technique is slow and places an undesirably thick layer of solder, often as thick as 1 mil, on the package leads. This thickness is much greater than is necessary for bonding and corrosion resistance and is often not as uniform as is desirable. The heat from the molten solder also may cause failure of an integrated circuit within the package or the bond between the metal leads and a plastic enclosure which encapsulates the integrated circuit.
The die receiving pad of a lead frame is typically spot plated with a precious metal. Spot plating is another type of selective plating. In spot plating apparatus, the anode typically comprises a pair of wires immersed in an electrolytic plating bath containing plating material. The bath fluid is forced through relatively small jets against the surface of the die pad. While uniform and controlled depositions can be obtained using a spot plating technique, the selective plating of larger areas using conventional spot plating techniques results in unstable electrolytic baths which must be constantly replenished with plating material and is therefore time consuming and costly.